The effects of estradiol and progesterone on plantarflexor muscle fatigue in ovariectomized mice.

The aim of this study was to examine specific and interactional effects of estradiol and progesterone on the time-to-fatigue of eccentrically contracted plantarflexor muscles and on the percent of plantarflexor isometric torque remaining immediately after an eccentric contraction (EC) protocol. Ovariectomized 6- to 8-week-old C57BL/6 mice were implanted with 21-day 0.05 mg-placebo, 0.05 mg-17-beta estradiol (OE), 15 mg-progesterone (OP), or estradiol and progesterone pellets (OEP). On the 16th day of hormone treatment, the isometric torque of the left plantarflexor muscles was measured. The left plantarflexor muscles then underwent 1 set of 150 ECs followed by 2 immediate post-EC isometric torque measurements. A group of ovarian-intact female mice of a similar age underwent the same isometric torque measurements and EC protocol. Plantarflexor muscle fatigue during ECs took 30%-41% longer to occur in the OP group (n = 9) than it did in the intact (n = 8, P = 0.02), OC (n = 11, P = 0.003), and OEP (n = 9, P = 0.007) groups. Peak active isometric torque had decreased immediately after ECs at 2 time points (M1 and M2). The OP group exhibited the greatest percent of isometric torque remaining immediately after ECs (M1, P = 0.03; M2, P = 0.04). These findings suggest that progesterone reduces muscle fatigue in response to ECs and that this progesterone effect is blunted when estradiol also is present. Therefore, ovarian hormone status may need to be considered when evaluating a response to physical activities, especially those activities involving ECs.